The invention relates to a cathode of individually exchangeable elements for a cell for fused salt electrolysis, in particular for the production of aluminum.
The production of aluminum from aluminum oxide by electrolysis is such that the latter is dissolved in a fluoride melt made up for the greater part of cryolite. The cathodic aluminum which separates out during the process collects under the fluoride melt on the carbon floor of the cell, the surface of the liquid aluminum itself forming the actual cathode. Anodes, which in conventional processes are made of amorphous carbon, are secured to overhead anode beams and dip into the melt from above. At the carbon anodes, as a result of the electrolytic decomposition of the aluminum oxide, oxygen is formed and combines with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in general at a temperature of about 940.degree.-970.degree. C. In the course of this process the electrolyte is depleted of aluminum oxide. At a lower concentration of about 1-2 wt.% aluminum oxide in the electrolyte the anode effect occurs, whereby an increase in voltage from e.g. 4-4.5 V to 30 V and higher occurs. Then at the latest the crust of solidified electrolyte must be broken open and the aluminum oxide concentration increased by the addition of fresh aluminum oxide (alumina).
In the fused salt electrolytic production of aluminum the use of cathodes which are wet by aluminum is well known. Also suggested for cathodes for state of the art electrolytic cells producing aluminum are cathodes made of titanium diboride, titanium carbide, pyrolytic graphite, boron carbide and other substances including mixtures of these substances which can be sintered together.
Compared with conventional electrolytic cells with an interpolar distance of ca. 6-6.5 cm, decisive advantages are offered by cathodes which can be wet with aluminum and are not soluble in aluminum or are only slightly soluble in aluminum. The cathodic precipitated aluminum flows on the cathode surface facing the active anode surface, even when the layer of deposited aluminum is very thin. It is possible, therefore, to conduct the precipitated, liquid aluminum out of the gap between the anode and cathode and to lead it to a sump away from this gap.
As a result of the thin aluminum layer on the cathode surface, non-uniformity in the thickness of the aluminum layer, due to electromagnetic and convection forces and well known from conventional electrolysis, does not occur. This means that the interpolar distance can be reduced without penalty in current efficiency i.e. as a result a much smaller energy consumption per unit metal produced is achieved.
Suggested in U.S. Pat. No. 3,400,061 is an electrolytic cell in which wettable cathodes are secured to the carbon floor of the cell. The cathode plates are slightly inclined to the horizontal, towards the center of the cell. The size of the gap between the anode and cathode, i.e. the interpolar distance, is much smaller than in conventional cells. This has the result, however, that it is more difficult for electrolyte to circulate between anode and cathode. As the aluminum precipitates out, the cryolite becomes strongly depleted in alumina which makes the cell susceptible to the anode effect. Only a small part of the cell floor area is available for collecting the liquid metal. Therefore, in order that the intervals between the tapping of the cell do not become uneconomically short, the sump must be made deep which again calls for extra insulation of the cell floor.
It should also be noted that the connection between the carbon floor and the wettable cathode plates requires properties of the adhesive mass which are difficult to achieve. This adhesive mass also increases the electrical resistance in the floor of the cell. As with conventional electrolytic cells the floor is made of electrically conductive, i.e. poor thermally insulating, carbonaceous material.
Wettable cathodes are also employed in the process according to the German patent application DE-OS No. 26 56 579. In that case the circulation of the cryolite melt is improved by having the cathode elements anchored in the electrically conductive cell floor and the region below the anodes projecting out of the aluminum sump which covers the rest of the cell floor area. The cathode elements in that case are pipes which are closed at the bottom, are full of aluminum, and are made of material which is wet by aluminum.
Above the aluminum sump i.e. between the pipes, gaps between the cathode elements make circulation of electrolyte easier. The height of these gaps or pipes is chosen such that there is no significant current flow between the anodes and the aluminum sump. The means of current supply to the cathode elements in the above mentioned example of the German patent application suffer from the disadvantage of having the current flowing through the carbon floor. The streaming of the electrolyte is a whirling action around the cathode elements without any preferential direction. This means that the alumina concentration pattern will not be optimal.
An extension to the above German patent application can be found in U.S. Pat. No. 4,177,128. The pipes, which are by choice of electrically conductive or non-conductive material, are provided with an exactly fitting lid made of electrically conductive material. This lid is connected via a downwards directed extension to the liquid aluminum in the pipe. According to this version, however, more titanium boride is used in the electrically conductive pipes than in the above mentioned German patent application; electrically insulating pipes are not adequately resistant towards the molten cryolite. Also sludge is formed in non-hermetically sealed pipes. The sludge is difficult to re-dissolve and is practically impossible to remove.
A basic disadvantage of all the versions with wettable cathodes discussed up to now is that these cathodes are all permanently anchored in the floor of the cell. For economic reasons, therefore, the material chosen for the wettable cathodes must be such that its service life is at least as long as or greater than the operational life of the cell lining. The use of a cheaper material with a shorter service life or a simpler method of manufacture would mean that the failure of only a small proportion of the cathode elements, for example because of mistakes in production or operation, would result in the whole cell being put out of service. The carbon floor with the cast-in cathode conductor bars is, in general, extremely sensitive to flaws introduced during its preparation.
The applicant has suggested therefore in U.S. Pat. No. 4,243,502 for a molten salt electrolytic cell, in particular such a cell for producing aluminum, a wettable cathode which comprises individually exchangeable elements each with at least one means of current supply. This type of easily exchangeable cathode element eliminates the most serious of the above mentioned disadvantages however, some of the inconveniences still remain. The electrically conductive, wettable elements are made of relatively expensive material which is difficult to shape. There are therefore limits to the size and geometric form of the elements.
It is therefore an object of the present invention to develop a cathode of individually exchangeable elements for a molten salt electrolytic cell for producing aluminum which, in particular in terms of shape and shaping, can be made more economically.